We have no idea what enemy we will be fighting in Halo 4. Keep watch for any new information released by 343 Industries.
It has been hinted that it is precursor tech... Here are a few possibilities:
Failed Flood Specimens- Since the precursors seem to have made the flood it makes sense that there would be leftovers.....
Precursor Robots- Since the Forerunners built sentinels the Precursors probobly had similar things.
Precursors- The "Timeless One" Was a lieing douchebag and there are still a bunch of Precursors out there.
Whatever they come up with- Whatever they think of and decide to put in.
That big alien thing in that one picture- It looks like a mutant Prophet that eats people... Its probobly gonna be there too.
Good Luck John.... Your gonna need it!
Just one more thing. Watch the halo 4 concept art trailer. At the end of the trailer there is a really fast picture and some sort of mechanical noise. It looks like the body of something so it could be a clue.
Other Unknown Alien Species- Since We Only Seen A Small Fraction Of The Halo Universe,There Maybe Other Undiscovered Sentient Species That Has Never Been Seen By The Covenant Or Humanity,So Whatever Other Species Exist, So They Could Also Pose A Threat.